The preferred embodiment relates to a method and device for communicating electrical positioning information of an actuator-positioned final control element such as a valve element. The positioning information is usually continually sensed with the aid of an analog sensor for implementing display and/or closed loop control of the position.
It is particularly in the field of process engineering and nuclear technology that final control valves are used in flow lines which, due to stringent safety requirements, are actuated by pneumatic actuators. These so-called field devices are usually also designed to automatically position in an emergency, for example, to close the valve element.
“Seeing” that this position really has been attained in an emergency is vital to safe operation. Known are alarm units such as the so-called namur contact as per the EN 60947-5-6 directive which is usually configured as an inductive proximity switch that transmits a binary signal to a display, by means of which the system can “see” whether the valve element has assumed the alarm position, i.e. the desired emergency position, or not.
Continually sensing the position for example by means of a Hall sensor has the known advantage over a simple alarm that closed loop control of positioning in accordance with the desired flow in the conduits and watchdogging the precise travel of the valve element in an emergency are possible. On the other hand, however, the simple alarm approach has the advantage of a low electrical energy requirement and safe functioning which is most welcome especially in an explosive hazard environment. To satisfy all safety aspects involved it would be a good thing to make available both a continual sensing of the position and an alarm for the field device concerned. Unfortunately this involves an expensive and complicated design.